In the real world, flight control systems operate in regimes where uncertain and unmodeled effects are encountered. In the hypersonic environment these effects may include variations in characteristics such as control surface effectiveness, aerodynamic parameters such as C.sub.m-q, airframe and/or control surface ablation, and so forth. Along with the uncertainties, design changes and/or in-flight changes to the airframe may shift the location of the flight vehicle's center-of-gravity, thereby changing its dynamic response characteristics. These uncertainties, coupled with a flight envelope which may vary the operating conditions from those at trajectory insertion (Mach 20, dynamic pressure of approximately 50 psf) to terminal maneuver operation (Mach 20, dynamic pressure of approximately 10,000 psf), place a great burden on the flexibility of the flight control system. It is necessary to have a flight control system, or autopilot, which can accommodate this wide range of operation.
To provide a flight control system for a hypersonic vehicle such as a hypersonic flight vehicle presents a special control problem. The traditional method of accommodating a wide range of operating conditions, configuration changes, and modeling uncertainties is to design an autopilot with a high degree of "robustness". This is often accomplished by arriving at pint designs for a cross section of operating conditions, using the well known 6-db gain and 30 degree phase margin requirements, and linking these point designs in-flight through gain and/or filter parameter variations based on some in-flight measurement, such as dynamic pressure or axial acceleration. The drawback to this control solution is that it requires a extensive outlay of resources up-front to design, analyze, and tune the autopilot to the particular vehicle configuration and application at hand. In fact, it is often difficult and sometimes impossible to arrive at a design which will accommodate complete spectrum of operation.
For the Hypersonic Glide Vehicle (HGV), this "spectrum of operation" includes a variety of phases. The Glide Phase is preceded by a high altitude, double digit mach number insertion, with possible high tip off rates and off nominal release conditions. During the Glide Phase the HGV will encounter altitude/density phugoids, and possible density bubbles, while attempting to fly a rather benign trajectory with small maneuver requirements. A phugoid oscillation is usually a lightly damped long period oscillation. However, in certain circumstances in supersonic flight, this oscillation may become unstable or may be replace by a subsidence and a divergence. At the onset of the Terminal Phase the vehicle requirements change drastically, as does the operating environment. For example, the dynamic pressure may change from 300 psf to 10,000 psf in a matter of minutes as the HGV transitions from the Glide Phase environment to the Terminal Phase region. A complicated maneuver such as a high-g/rolling maneuver to avoid a threat, may then be performed as the HGV proceeds enroute to a target.
Because of the wide operating range requirement, an HGV autopilot design which somehow adapts to changing operation conditions seems a desirable choice. In this context an adaptive autopilot is one which measures the response of the vehicle to a known excitation signal and uses these measurements to control certain autopilot parameters.
The HGV adaptive control design must satisfy certain ground rules. It must be compatible with the existing airframe without structural modification. The autopilot is also constrained to utilize existing control surfaces (elevons and rudder) and sensors currently envisioned for the HGV. Due to mission concerns, any control surface excitation signals must be of low frequency and small amplitude to conserve battery power throughout the entire flight. Computer cycle time is not considered to be a major design concern in light of the rate of advance in computer capability.
The prior art reveals two patents that are of general interest only. U.S. Pat. No. 4,122,448 issued Oct. 4, 1978 for an AUTOMATIC PHASE AND GAIN CONTROLLER FOR A BASEBAND PROCESSOR relates to a baseband processor for a moving target indicator (MTI) type radar system that includes an automatic phase and gain balance controller that utilizes a pilot signal for separately sensing phase and magnitude errors representative of the unbalance in the baseband processor channels which generate the inphase and quadrature components and nulling these sensed errors by providing balancing adjustments in the channels. This patent relates only to adjustments in a radar system and does not relate to the control of the parameters of an autopilot system for a hypersonic flight vehicle as seen in the present invention.
U.S. Pat. No. 4,129,275 issued Dec. 12, 1978 for an AUTOMATIC FLIGHT CONTROL APPARATUS FOR AIRCRAFT discloses a flight control system which uses a reference signal representing the desired attitude of the aircraft in a balanced relation to the actual air speed thereof and using the difference signal between the reference signal and a signal representing the actual attitude of the aircraft as one of the control parameters. The disclosed flight control apparatus also teaches a limiter for preventing unstable flight which would be caused by the feeding of an excessively large difference signal to the control system of the aircraft.